1. Field of the Invention
This invention relates to selected light-sensitive 1,2-naphthoquinone-2-diazide-4-sulfonic acid monoester compounds. Moreover, the present invention relates to light-sensitive mixtures useful as positive-working photoresist compositions containing alkali-soluble resins together with these 1,2-naphthoquinone-2-diazide-4-sulfonic acid monoester light-sensitive compounds as well as substrates coated therewith.
2. Description of Related Art
Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Ultraviolet (UV) light and electron beam energy are radiation sources commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions--negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become insoluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain soluble to a developing solution. Thus, treatment of an exposed negative-working resist with the developer causes removal of the non-exposed areas of the resist coating and the creation of a negative image in the photoresist coating. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become soluble to the developer solution (e.g. a decomposition reaction occurs) while those areas not exposed remain insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating.
After this development operation, the partially unprotected substrate is treated with a substrate-etchant solution or plasma gases or the like. This etchant solution or plasma gases etches the portion of the substrate where the photoresist coating was previously removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the image-wise exposure of the actinic radiation. Later, the remaining areas of the photoresist coating are removed during a stripping operation, leaving a clean etched substrate surface.
Positive-working photoresist compositions are currently favored over negative-working photoresists because the former generally have better resolution and pattern transfer techniques.
Photoresist resolution is the lower limit of geometrical equal line and space patterns which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of one micron or less are necessary.
In addition, it is generally desirable that the developed photoresist profile be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate after etching.
Several ingredients commonly make up positive photoresist compositions. A light stable, water-insoluble, alkali-soluble and film-forming resin (or mixture of resins) is usually the major solid component. Phenol-formaldehyde novolaks and cresol-formaldehyde novolaks and polyvinyl phenols are well known examples of such resins. One or more light-sensitive compounds (also known as photoactive compounds or sensitizers) are also present in the photoresist composition. Naphthoquinone diazide compounds are examples of such sensitizers. When a film is formed from this resin component it is soluble in an aqueous alkaline developing solution. However, addition of the sensitizer inhibits the dissolution of the film in the developing solution. When the substrate coated with the positive-working photoresist composition is subjected to an image-wise exposure of radiation, the sensitizer undergoes a radiation-induced chemical transformation in those exposed areas of the coating. This photochemical transformation eliminates the solubility-inhibiting property that the sensitizer had on the film-forming resin in alkaline developers. Accordingly, the radiation-exposed areas of the coating are now more soluble to aqueous alkaline developing solutions than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to dissolve when the coated substrate is immersed in the aqueous alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
Among the substituted naphthoquinone diazide sensitizers known are the 1,2-naphthoquinone-2-diazide-5 or 4-sulfonic acid monoesters of cyclohexanol [See U.S. Pat. No. 2,767,092 (Schmidt)], substituted benzenes [See U.S. Pat. Nos. 3,106,465 (Neugebauer); 3,130,047 (Uhlig et al) and 3,640,992 (Sus et al)] and substituted cyclohexanols [See U.S. Pat. No. 3,823,130 (Deutsch et al)].
Positive-working photoresist compositions preferably contain other ingredients besides the film-forming resin and sensitizer. For example, one or more solvents may be added. Ethyl lactate, ethylene glycol methyl ether acetate, propylene glycol methyl ether acetate, N-methyl-2-pyrrolidone, and mixtures of ethyl cellosolve acetate, butyl acetate and xylene are examples of commonly used solvents. The resin and sensitizer are dissolved in the solvent or solvents to facilitate their application to the substrate.
Other preferred photoresist additives include actinic and contrast dyes, anti-striation agents, plasticizers, speed enhancers and the like. Actinic dyes help provide increased resolution by inhibiting back scattering of light off the substrate. This back scattering causes the undesirable effect of optical notching, especially where the substrate surface is highly reflective or has topography. Contrast dyes enhance the visibility of developed images and facilitate pattern alignment during manufacturing. Anti-striation agents level out the photoresist coating or film to a uniform thickness. This is important to ensure uniform radiation exposure over the film surface. Plasticizers improve the coating and adhesion properties of the photoresist composition and better allow for the application of a thin coating or film of photoresist which is smooth and of uniform thickness onto the substrate. Speed enhancers tend to increase the solubility of the photoresist coating in both the exposed and unexposed areas, and thus, they are used in applications where speed of development is the overriding consideration even though some degree of contrast may be sacrificed, i.e. while the exposed areas of the photoresist coating will be dissolved more quickly by the developer, the speed enhancers will also cause a larger loss of photoresist coating from the unexposed areas.
In all, while numerous types of sensitizers have been employed in making positive-working photoresists, there is still a need to find new ones which may be better with specific film-forming resins or with specific solvents or additives for specific processing applications. The present invention is thus directed to a new class of light-sensitive compounds which may have advantages in such situations.